1. Field of the Invention
The present invention relates to a defect evaluation method for a wide bandgap semiconductor using a CPM measurement apparatus.
2. Description of the Related Art
Although transistors formed using amorphous silicon had been used for conventional display devices typified by liquid crystal televisions, an oxide semiconductor has attracted attention as a material which replaces a silicon semiconductor in order to form transistors. For example, an active matrix display device in which an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is used for an active layer of a transistor and the electron carrier concentration of the amorphous oxide is lower than 1×1018 cm−3 is disclosed (see Patent Document 1).
However, some problems of a transistor using an oxide semiconductor have been pointed out. One of them is instability of characteristics, and it is pointed out that the threshold voltage is changed by irradiation with visible light and ultraviolet light (see Non-Patent Document 1). Further, in terms of reliability of a transistor, a problem in that characteristics are changed by a bias-temperature stress test has been pointed out (see Non-Patent Document 2).